1. Field of the Invention
The present invention relates to a substrate having a conductive layer thereon and a method of manufacturing such a substrate. The conductive layer is formed by laminating different kinds of conductive metal layers, and has a predetermined shape. The present invention can be effectively adopted to, for example, a metal substrate for a hybrid integrated circuit (IC).
2. Description of the Related Art
A conventional metal substrate for a hybrid IC in which an electric circuit pattern is formed on an insulating layer has been known. The metal substrate is formed by the following process. First the insulating layer of resin and then a conductive metal layer formed of a plurality of sublayers are formed on a metal plate. The metal layer is then etched to form the pattern.
Such a substrate uses a laminated body formed by laminating different kinds of conductive metal sublayers as the metal layer, one of the sublayers is of a material to improve an electric conductivity, and a thermal conductivity, and another is of a material to improve a wire bonding characteristics. An example of such a laminated body is disclosed in, for example, U.S. Pat. No. 4,521,476.
The laminated body is formed, for example, as shown in FIG. 8(a). Namely, a zinc (Zn) layer is formed on rolled aluminum foil by a substitution plating method. A copper (Cu) layer is then formed on the zinc layer by an electroplating method. Thus, the laminated body has an Al-Zn-Cu structure. The copper layer is bonded to an insulating layer. According to this structure, electric conductivity and thermal conductivity of the laminated body are improved because of the copper layer, and wire bonding characteristic to the laminated body is also improved because of the aluminum foil.
However, when a hybrid IC substrate having the above-mentioned laminated body has been operated in a damp atmosphere for a long time, some of the metal existing at the contact portion between the laminated metal layers becomes eluted. "Eluted" means that a part of the metal is actually lost so that the structure becomes as shown in FIG. 8(b), with the Zn being shorter than the other metals. It is believed that this is caused by a water film being formed on a surface of the laminated body, and the specific metal being ionized due to the potential difference between the laminated metal layers. For example, in the laminated body shown in FIG. 8(a), zinc is ionized, and the ionized zinc becomes as shown in FIG. 8(b). Therefore, the upper layer of aluminum foil more easily comes off the copper layer, because the zinc layer, which bonds the aluminum foil to the copper layer, is decreased in size.
Generally speaking, a larger difference between ionization tendencies of the conductive metal layers contacting each other will cause more metal to be eluted. Moreover, the eluted metal is the metal has a larger ionization tendency. In the laminated body shown in FIG. 8(a), the difference between ionization tendencies of copper and zinc is larger than between zinc and aluminum, and a stable alumina (Al.sub.2 O.sub.3) layer has been formed on the surface of the aluminum foil. Therefore, aluminum which has the largest ionization tendency among these metals is not eluted, and instead zinc is eluted.
The elution length L (shown in FIG. 8(b)) increases over time. Especially when the insulating layer is a resin layer, such as an epoxy layer, the elution length L becomes long, because is that chlorine ions (Cl-) exist as free ions in the resin layer and they change the water film into an electrolytic water solution film.